Semiconductor device, memory system and electronic apparatus

ABSTRACT

A semiconductor device having a memory cell including first and second load transistors, first and second driver transistors, and first and second transfer transistors. The semiconductor device includes first and second gate-gate electrode layers, first and second drain-drain wiring layers, and first and second drain-gate wiring layers. The first drain-gate wiring layer and the second drain-gate wiring layer are located in different layers. The first drain-gate wiring layer is located below the first drain-drain wiring layer, and the second drain-gate wiring layer is located in above the first drain-drain wiring layer. This structure provides a semiconductor device that has reduced cell area. The invention also provides a memory system and electronic apparatus that include the above semiconductor device.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to semiconductor devices, such as, for example, SRAMs (static random access memories), and memory systems and electronic apparatuses equipped with the same.

[0003] 2. Description of the Related Art

[0004] SRAMs are a type of semiconductor memory devices that do not require a refreshing operation and therefore have properties that can simplify the system and lower power consumption. For this reason, the SRAMs are widely used as memories for electronic equipment, such as mobile phones.

SUMMARY OF THE INVENTION

[0005] The present invention provides semiconductor devices that can reduce cell area. The present invention also provides memory systems and electronic apparatuses that include such semiconductor devices.

[0006] Additional features and advantages of the invention will be set forth in the descriptions that follow and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims thereof as well as the appended drawings.

[0007] To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, the present invention provides a semiconductor device having a memory cell including a first load transistor, a second load transistor, a first driver transistor, a second driver transistor, a first transfer transistor, and a second transfer transistor, the semiconductor device includes a first gate-gate electrode layer including a gate electrode of the first load transistor and a gate electrode of the first driver transistor; a second gate-gate electrode layer including a gate electrode of the second load transistor and a gate electrode of the second driver transistor; a first drain-drain wiring layer that forms a part of a connection layer that electrically connects a drain of the first load transistor and a drain of the first driver transistor; a second drain-drain wiring layer that forms a part of a connection layer that electrically connects a drain of the second load transistor and a drain of the second driver transistor; a first drain-gate wiring layer that forms a part of a connection layer that electrically connects the first gate-gate electrode layer and the second drain-drain wiring layer; and a second drain-gate wiring layer that forms a part of a connection layer that electrically connects the second gate-gate electrode layer and the first drain-drain wiring layer. The first drain-gate wiring layer and the second drain-gate wiring layer are located in different layers. The first drain-gate wiring layer is located in a layer below the first drain-drain wiring layer, and the second drain-gate wiring layer is located in a layer above the first drain-drain wiring layer.

[0008] Because the first drain-gate wiring layer and the second drain-gate wiring layer are located in different layers, the pattern density of each of the wiring layers where the first drain-gate wiring layer and the second drain-gate wiring layer are formed can be reduced, compared to the case where the first drain-gate wiring layer and the second drain-gate wiring layer are formed in the same layer. As a result, the cell area can be made smaller.

[0009] In another aspect, the present invention provides a memory system including the above semiconductor device.

[0010] In yet another aspect, the present invention provides an electronic apparatus including the above semiconductor device.

[0011] It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 shows an equivalent circuit of an SRAM in accordance with an embodiment of the present invention.

[0013]FIG. 2 schematically shows a plan view of a field of the memory cell of the SRAM in accordance with the present embodiment.

[0014]FIG. 3 schematically shows a plan view of a first conductive layer of the memory cell of the SRAM in accordance with the present embodiment.

[0015]FIG. 4 schematically shows a plan view of a second conductive layer of the memory cell of the SRAM in accordance with the present embodiment.

[0016]FIG. 5 schematically shows a plan view of a third conductive layer of the memory cell of the SRAM in accordance with the present embodiment.

[0017]FIG. 6 schematically shows a plan view of a fourth conductive layer of the memory cell of the SRAM in accordance with the present embodiment.

[0018]FIG. 7 schematically shows a plan view of the field and the first conductive layer of the memory cell of the SRAM in accordance with the present embodiment.

[0019]FIG. 8 schematically shows a plan view of the field and the second conductive layer of the memory cell of the SRAM in accordance with the present embodiment.

[0020]FIG. 9 schematically shows a plan view of the first conductive layer and the second conductive layer of the memory cell of the SRAM in accordance with the present embodiment.

[0021]FIG. 10 schematically shows a plan view of the second conductive layer and the third conductive layer of the memory cell of the SRAM in accordance with the present embodiment.

[0022]FIG. 11 schematically shows a plan view of the first conductive layer and the third conductive layer of the memory cell of the SRAM in accordance with the present embodiment.

[0023]FIG. 12 schematically shows a plan view of the third conductive layer and the fourth conductive layer of the memory cell of the SRAM in accordance with the present embodiment.

[0024]FIG. 13 schematically shows a cross-sectional view taken along a line A-A shown in FIG. 2-FIG. 6.

[0025]FIG. 14 schematically shows a cross-sectional view taken along a line B-B shown in FIG. 2-FIG. 6.

[0026]FIG. 15 shows a block diagram of a part of a mobile telephone system equipped with the SRAM in accordance with the present embodiment.

[0027]FIG. 16 shows a perspective view of a mobile telephone that is equipped with the mobile telephone system shown in FIG. 15.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0028] An embodiment of the present invention is described. The present embodiment is the one in which a semiconductor device in accordance with the present invention is applied to in an SRAM.

[0029] [1] Equivalent Circuit of an SRAM

[0030]FIG. 1 shows an equivalent circuit of an SRAM in accordance with the present embodiment. The SRAM of the present embodiment is a type in which one memory cell is formed with six MOS field effect transistors. One CMOS inverter is formed by an n-channel type driver transistor Q3 and a p-channel type load transistor Q5. Another CMOS inverter is formed by an n-channel type driver transistor Q4 and a p-channel type load transistor Q6. These two CMOS inverters are cross-coupled to form a flip-flop as shown. Further, one memory cell is formed from this flip-flop and n-channel type transfer transistors Q1 and Q2.

[0031] [2] Structure of the SRAM

[0032] A structure of the SRAM is described below. The SRAM is formed including an element forming region formed in a field, a first conductive layer, a second conductive layer, a third conductive layer, and a fourth conductive layer. The structure of each of the field, and the first through fourth conductive layers is described below.

[0033] (1) Field

[0034] Referring to FIG. 2, the field is described. The field includes first through fourth active regions 14, 15, 16 and 17, and an element isolation region 12. The first through fourth active regions 14, 15, 16 and 17 are defined by the element isolation region 12. A region on the side where the first and second active regions 14 and 15 are formed is an n-type well region W10, and a region on the side where the third and fourth active regions 16 and 17 are formed is a p-type well region W20. The first active region 14 and the second active region 15 are disposed in a symmetrical relation in their plane configuration, and the third active region 16 and the fourth active region 17 are disposed in a symmetrical relation in their plane configuration.

[0035] The first load transistor Q5 is formed in the first active region 14. In the first active region 14, a first p⁺-type impurity layer 14 a and a second p⁺-type impurity layer 14 b are formed. The first p⁺-type impurity layer 14 a functions as a source of the first load transistor Q5. The second p⁺-type impurity layer 14 b functions as a drain of the first load transistor Q5. The second load transistor Q6 is formed in the second active region 15. In the second active region 15, a third p⁺-type impurity layer 15 a and a fourth p⁺-type impurity layer 15 b are formed. The third p⁺-type impurity layer 15 a functions as a source of the second load transistor Q6. The fourth p⁺-type impurity layer 15 b functions as a drain of the second load transistor Q6.

[0036] The first driver transistor Q3 and the first transfer transistor Q1 are formed in the third active region 16. In the third active region 16, first through third n⁺-type impurity layers 16 a, 16 b and 16 c that are to become components of the transistors Q1 and Q3, and a fifth p⁺-type impurity layer 16 d that constitutes a well contact region are formed. The first n⁺-type impurity layer 16 a functions as a source or a drain of the first transfer transistor Q1. The second n⁺-type impurity layer 16 b functions as a drain of the first driver transistor Q3, and a drain or a source of the first transfer transistor Q1. The third n⁺-type impurity layer 16 c functions as a source of the first driver transistor Q3.

[0037] The second driver transistor Q4 and the second transfer transistor Q2 are formed in the fourth active region 17. In the fourth active region 17, fourth through sixth n⁺-type impurity layers 17 a, 17 b and 17 c that are to become components of the transistors Q2 and Q4, and a sixth p⁺-type impurity layer 17 d that constitutes a well contact region are formed. The fourth n⁺-type impurity layer 17 a functions as a source or a drain of the second transfer transistor Q2. The fifth n⁺-type impurity layer 17 b functions as a drain of the second driver transistor Q4, and a drain or a source of the second transfer transistor Q2. The sixth n⁺-type impurity layer 17 c functions as a source of the second driver transistor Q4.

[0038] (2) First Conductive Layer

[0039] Next, referring to FIG. 3 and FIG. 7, the first conductive layer is described. The first conductive layer is formed on the semiconductor substrate.

[0040] The first conductive layer includes a first gate-gate electrode layer 20, a second gate-gate electrode layer 22, a first drain-gate wiring layer 30 and an auxiliary word line 24. The first gate-gate electrode layer 20 and the second gate-gate electrode layer 22 are formed in a manner to extend along a Y direction shown in the figures. The first drain-gate wiring layer 30 and the auxiliary word line 24 are formed in a manner to extend along an X direction shown in the figures. Here, a “wiring layer” refers to a conductive layer in a layer disposed on a field or an interlayer dielectric layer. Components of the first conductive layer are described concretely below.

[0041] 1) First Gate-Gate Electrode Layer

[0042] The first gate-gate electrode layer 20 is formed transverse to the first active region 14 and the third active region 16, as shown in FIG. 7. The first gate-gate electrode layer 20 functions as a gate electrode of the first load transistor Q5 and the first driver transistor Q3. The first gate-gate electrode layer 20 is formed in a manner to pass between the first p⁺-type impurity layer 14 a and the second p⁺-type impurity layer 14 b, in the first active region 14. Thus, the first gate-gate electrode layer 20, the first p⁺-type impurity layer 14 a and the second p⁺-type impurity layer 14 b form the first load transistor Q5. Similarly, the first gate-gate electrode layer 20 is formed in a manner to pass between the second n⁺-type impurity layer 16 b and the third n⁺-type impurity layer 16 c, in the third active region 16. Thus, the first gate-gate electrode layer 20, the second n⁺-type impurity layer 16 b and the third n⁺-type impurity layer 16 c form the first driver transistor Q3.

[0043] 2) Second Gate-Gate Electrode Layer

[0044] The second gate-gate electrode layer 22 is formed transverse to the second active region 15 and the fourth active region 17, as shown in FIG. 7. The second gate-gate electrode layer 22 functions as a gate electrode of the second load transistor Q6 and the second driver transistor Q4. The second gate-gate electrode layer 22 is formed in a manner to pass between the third p⁺-type impurity layer 15 a and the fourth p⁺-type impurity layer 15 b, in the second active region 15. Thus, the second gate-gate electrode layer 22, the third p⁺-type impurity layer 15 a and the fourth p⁺-type impurity layer 15 b form the second load transistor Q6. Similarly, the second gate-gate electrode layer 22 is formed in a manner to pass between the fifth n⁺-type impurity layer 17 b and the sixth n⁺-type impurity layer 17 c, in the fourth active region 17. Thus, the second gate-gate electrode layer 22, the fifth n⁺-type impurity layer 17 b and the sixth n⁺-type impurity layer 17 c form the second driver transistor Q4.

[0045] 3) First Drain-Gate Wiring Layer

[0046] The first drain-gate wiring layer 30 is formed in a manner to extend in the X direction from a side section of the first gate-gate electrode layer 20 toward the second gate-gate electrode layer 22. Also, as shown in FIG. 7, the first drain-gate wiring layer 30 is formed at least between the first active region 14 and the third active region 16. The plan configuration of the first drain-gate wiring layer 30 may be in a linear configuration.

[0047] 4) Auxiliary Word Line

[0048] The auxiliary word line 24 is formed transverse to the third active region 16 and the fourth active region 17, as shown in FIG. 7. The auxiliary word line 24 functions as a gate electrode of the first and second transfer transistors Q1 and Q2. The auxiliary word line 24 is formed in a manner to pass between the first n⁺-type impurity layer 16 a and the second n⁺-type impurity layer 16 b, in the third active region 16. Thus, the auxiliary word line 24, the first n⁺-type impurity layer 16 a and the second n⁺-type impurity layer 16 b form the first transfer transistor Q1. Similarly, the auxiliary word line 24 is formed in a manner to pass between the fourth n⁺-type impurity layer 17 a and the fifth n⁺-type impurity layer 17 b, in the fourth active region 17. Thus, the auxiliary word line 24, the fourth n⁺-type impurity layer 17 a and the fifth n⁺-type impurity layer 17 b form the second transfer transistor Q2.

[0049] 5) Cross-Sectional Structure of First Conductive Layer and Others

[0050] The first conductive layer may be formed by successively depositing a polysilicon layer and a silicide layer in layers. As shown in FIG. 13 and FIG. 14, a first interlayer dielectric layer 90 is formed on the field (e.g. 17 a and 17 b) and the first conductive layer (e.g. 30 and 24). The first interlayer dielectric layer 90 may be formed through a planarization process utilizing, for example, a chemical mechanical polishing method.

[0051] (3) Second Conductive Layer

[0052] Referring to FIG. 4, FIG. 8 and FIG. 9, the second conductive layer is described below. The second conductive layer is formed on the first interlayer dielectric layer 90 (see FIGS. 13 and 14).

[0053] The second conductive layer includes, as shown in FIG. 4, a first drain-drain wiring layer 40, a second drain-drain wiring layer 42, a first BL contact pad layer 70 a, a first /BL contact pad layer 72 a, a first Vss contact pad layer 74 a and a Vdd contact pad layer 76. The first drain-drain wiring layer 40 and the second drain-drain wiring layer 42 are formed in a manner to extend along the Y direction. Components of the second conductive layer are concretely described below.

[0054] 1) First Drain-Drain Wiring Layer

[0055] The first drain-drain wiring layer 40 has portions that overlap the first active region 14 and the third active region 16 as viewed in a plan view, as shown in FIG. 8. Specifically, one end section of the first drain-drain wiring layer 40 is located above the second p⁺-type impurity layer 14 b. The one end section of the first drain-drain wiring layer 40 and the second p⁺-type impurity layer 14 b are electrically connected to each other through a contact section between the field and the second conductive layer (herein below referred to as a “field/second-layer contact section”) 80. The other end section of the first drain-drain wiring layer 40 is located above the second n⁺-type impurity layer 16 b. The other end section of the first drain-drain wiring layer 40 and the second n⁺-type impurity layer 16 b are electrically connected to each other through another field/second-layer contact section 80.

[0056] 2) Second Drain-Drain Wiring Layer

[0057] The second drain-drain wiring layer 42 has portions that overlap the second active region 15 and the fourth active region 17 as viewed in a plan view, as shown in FIG. 8. Specifically, one end section of the second drain-drain wiring layer 42 is located above the fourth p⁺-type impurity layer 15 b. The one end section of the second drain-drain wiring layer 42 and the fourth p⁺-type impurity layer 15 b are electrically connected to each other through a field/second-layer contact section 80. The other end section of the second drain-drain wiring layer 42 is located above the fifth n⁺-type impurity layer 17 b. The other end section of the second drain-drain wiring layer 42 and the fifth n⁺-type impurity layer 17 b are electrically connected to each other through another field/·second-layer contact section 80.

[0058] Further, the second drain-drain wiring layer 42 has a portion that overlaps an end section of the first drain-gate wiring layer 30 as viewed in a plan view, as shown in FIG. 9. The second drain-drain wiring layer 42 and the end section of the first drain-gate wiring layer 30 are electrically connected to each other through a contact section between the first conductive layer and the second conductive layer (hereafter referred to as a “first-layer/second-layer contact section”) 82.

[0059] 3) First BL Contact Pad Layer

[0060] The first BL contact pad layer 70 a is located above the first n⁺-type impurity layer 16 a in the third active region 16, as shown in FIG. 8. The first BL contact pad layer 70 a and the first n⁺-type impurity layer 16 a are electrically connected to each other through a field/second-layer contact section 80.

[0061] 4) First/BL Contact Pad Layer

[0062] The first /BL contact pad layer 72 a is located above the fourth n⁺-type impurity layer 17 a in the fourth active region 17, as shown in FIG. 8. The first/BL contact pad layer 72 a and the fourth n⁺-type impurity layer 17 a are electrically connected to each other through a field/second-layer contact section 80.

[0063] 5) First Vss Contact Pad Layer

[0064] The first Vss contact pad layers 74 a are located above the sources of the driver transistors Q3 and Q4 (for example, the third n⁺-type impurity layer 16 c) and the well contact region (for example, the fifth p⁺-type impurity layer 16 d). Each of the first Vss contact pad layers 74 a is electrically connected to the source of each of the driver transistors Q3 and Q4 (for example, the third n⁺-type impurity layer 16 c) through a field/second-layer contact section 80. Also, each of the first Vss contact pad layers 74 a is electrically connected to the well contact region (for example, the fifth p⁺-type impurity layer 16 d) through another field/second-layer contact section 80.

[0065] 6) Vdd Contact Pad Layer

[0066] Each of the Vdd contact pad layers 76 is located above the source (for example, the first p⁺-type impurity layer 14 a) of each of the load transistors Q5 and Q6. Each of the Vdd contact pad layers 76 is electrically connected to the source (for example, the first p⁺-type impurity layer 14 a) of each of the load transistors Q5 and Q6 through a field/second-layer contact section 80.

[0067] 7) Cross-Sectional Structure of Second Conductive Layer

[0068] Next, a cross-sectional structure of the second conductive layer is described with reference to FIG. 13 and FIG. 14. The second conductive layer is preferably formed from, for example, a layer of nitride of a high melting point metal. The thickness of the second conductive layer may be preferably for example 100-200 nm, and more preferably be 140-160 nm. The layer of nitride of a high melting point metal may be formed from, for example, titanium nitride. Because the second conductive layer is formed from a layer of nitride of a high melting point metal, the thickness of the second conductive layer can be made smaller, and miniature processing thereof can be readily conducted. Accordingly, the cell area can be reduced.

[0069] Also, the second conductive layer may have either one of the following structures. a) It may have a structure in which a layer of nitride of a high melting point metal is formed on a metal layer formed from a high melting point metal. In this case, the metal layer formed from a high melting point metal is an under-layer, and may be formed of a titanium layer, for example. Titanium nitride may be used as a material of the layer of nitride of a high melting point metal. b) The second conductive layer may be formed of a metal layer of a high melting point metal alone.

[0070] Next, a cross-sectional structure of a field/second-layer contact section 80 is described with reference to FIG. 14. The field/second-layer contact section 80 is formed in a manner to fill a through hole 90 a that is formed in the first interlayer dielectric layer 90. The field/second-layer contact section 80 includes a barrier layer 80 a, and a plug 80 b formed over the barrier layer 80 a. Titanium and tungsten may be used as material of the plugs. The barrier layer 80 a may preferably be formed from a metal layer of a high melting point metal, and a layer of nitride of a high melting point metal formed over the metal layer. For example, titanium may be used as material of the metal layer of a high melting point metal. Titanium nitride, for example, may be used as material of the layer of nitride of a high melting point metal.

[0071] Next, a cross-sectional structure of a first-layer/second-layer contact section 82 is described with reference to FIG. 13 and FIG. 14. The first-layer/second-layer contact section 82 is formed in a manner to fill a through hole 90 b that is formed in the first interlayer dielectric layer 90. The first-layer/second-layer contact section 82 may have the same structure as that of the field/second-layer contact section 80 described above.

[0072] A second interlayer dielectric layer 92 is formed in a manner to cover the second conductive layer, as shown in FIGS. 13 and 14. The second interlayer dielectric layer 92 may be formed through a planarization process using, for example, a chemical mechanical polishing method.

[0073] (4) Third Conductive Layer

[0074] The third conductive layer is described below with reference to FIG. 5, FIG. 10 and FIG. 11. The third conductive layer is formed on the second interlayer dielectric layer 92 (see FIGS. 13 and 14).

[0075] As shown in FIG. 5, the third conductive layer includes a second drain-gate wiring layer 32, a main word line 50, a Vdd wiring 52, a second BL contact pad layer 70 b, a second/BL contact pad layer 72 b and a second Vss contact pad layer 74 b. The second drain-gate wiring layer 32, the main word line 50 and the Vdd wiring 52 are formed in a manner to extend along the X direction. The second BL contact pad layer 70 b, the second/BL contact pad layer 72 b and the second Vss contact pad layer 74 b are formed in a manner to extend in the Y direction. Components of the third conductive layer are concretely described below.

[0076] 1) Second Drain-Gate Wiring Layer

[0077] The second drain-gate wiring layer 32 is formed transverse to the second drain-drain wiring layer 42 in the second conductive layer, as shown in FIG. 10. Specifically, as shown in FIGS. 10 and 11, the second drain-gate wiring layer 32 is formed from an area above the first drain-drain wiring layer 40 in the second conductive layer to an area above the second gate-gate electrode layer 22 in the first conductive layer. Also, the second drain-gate wiring layer 32 is formed over the first drain-gate wiring layer 30. The second drain-gate wiring layer 32 may be in a linear configuration in its plan configuration.

[0078] The second drain-gate wiring layer 32 is, as shown in FIG. 10, electrically connected to the first drain-drain wiring layer 40 through a contact section between the second conductive layer and the third conductive layer (herein after referred to as a “second-layer/third-layer contact section”) 84. Also, the second drain-gate wiring layer 32 is, as shown in FIG. 11, electrically connected to the second gate-gate electrode layer 22 through a contact section between the first conductive layer and the third conductive layer (herein after referred to as a “first-layer/third-layer contact section”) 88. Thus, as shown in FIG. 1, the first drain-drain wiring layer 40 in the second conductive layer and the second gate-gate electrode layer 22 in the first conductive layer are electrically connected to each other through the second-layer/third-layer contact section 84, the second gate-drain wring layer 32, and the first-layer/third-layer contact section 88.

[0079] 2) Vdd Wiring

[0080] The Vdd wiring 52 is formed in a manner to pass over the Vdd contact pad layer 76, as shown in FIG. 10. The Vdd wiring 52 is electrically connected to the Vdd contact pad layer 76 through the second-layer/third-layer contact section 84.

[0081] 3) Second BL Contact Pad Layer

[0082] The second BL contact pad layer 70 b is located above the first BL contact pad layer 70 a, as shown in FIG. 10. The second BL contact pad layer 70 b is electrically connected to the first BL contact pad layer 70 a through a second-layer/third-layer contact section 84.

[0083] 4) Second/BL Contact Pad Layer

[0084] The second/BL contact pad layer 72 b is located above the first/BL contact pad layer 72 a, as shown in FIG. 10. The second/BL contact pad layer 72 b is electrically connected to the first/BL contact pad layer 72 a through a second-layer/third-layer contact section 84.

[0085] 5) Second Vss Contact Pad Layer

[0086] The second Vss contact pad layer 74 b is located above the first Vss contact pad layer 74 a, as shown in FIG. 10. The second Vss contact pad layer 74 b is electrically connected to the first Vss contact pad layer 74 a through a second-layer/third-layer contact section 84.

[0087] 6) Cross-Sectional Structure of Third Conductive Layer

[0088] Next, a cross-sectional structure of the third conductive layer is described with reference to FIG. 13 and FIG. 14. The third conductive layer has a structure in which, for example, a layer of nitride of a high melting point metal, a metal layer, and a layer of nitride of a high melting point metal, in this order from the bottom, are successively stacked in layers. For example, titanium nitride may be used as material of the layer of nitride of a high melting point metal. Aluminum, copper or an alloy of these metals, for example, may be used as material of the metal layer.

[0089] Next, a cross-sectional structure of a second-layer/third-layer contact section 84 is described. The second-layer/third-layer contact section 84 is, as shown in FIGS. 13 and 14, formed in a manner to fill a through hole 92 a formed in the second interlayer dielectric layer 92. The second-layer/third-layer contact section 84 may be provided with the same structure as that of the field/second-layer contact section 80 described above.

[0090] Next, a cross-sectional structure of a first-layer/third-layer contact section 88 is described. The first-layer/third-layer contact section 88 is formed in a manner to fill a through hole 92 b that is formed in the first interlayer dielectric layer 90 and the second interlayer dielectric layer 92, as shown in FIG. 13. The first-layer/third-layer contact section 88 may have the same structure as that of the field/second-layer contact section 80 described above.

[0091] A third interlayer dielectric layer 94 is formed in a manner to cover the third conductive layer, as shown in FIGS. 13 and 14. The third interlayer dielectric layer 94 may be formed through a planarization process using, for example a chemical mechanical polishing method.

[0092] (5) Fourth Conductive Layer

[0093] The fourth conductive layer is described below with reference to FIG. 6 and FIG. 12. The fourth conductive layer is formed on the third interlayer dielectric layer 94 (see FIGS. 13 and 14). The fourth conductive layer includes a bit line 60, a /bit line 62 and a Vss wiring 64. The bit line 60, the /bit line 62 and the Vss wiring 64 are formed in a manner to extend along the Y direction.

[0094] 1) Bit Line

[0095] The bit line 60 is formed in a manner to pass over the second BL contact pad layer 70 b, as shown in FIG. 12. The bit line 60 is electrically connected to the second BL contact pad layer 70 b through a contact section between the third conductive layer and the fourth conductive layer (herein below referred to as a “third-layer/fourth-layer contact section”) 86.

[0096] 2)/Bit Line

[0097] The /bit line 62 is formed in a manner to pass over the second/BL contact pad layer 72 b, as shown in FIG. 12. The /bit line 62 is electrically connected to the /second BL contact pad layer 72 b through a third-layer/fourth-layer contact section 86.

[0098] 3) Vss Wiring

[0099] The Vss wiring 64 is formed in a manner to pass over the second Vss contact pad layer 74 b, as shown in FIG. 12. The Vss wiring 64 is electrically connected to the second Vss contact pad layer 74 b through a third-layer/fourth-layer contact section 86.

[0100] 4) Cross-Sectional Structure of Fourth Conductive Layer

[0101] Next, a cross-sectional structure of the fourth conductive layer is described with reference to FIG. 13 and FIG. 14. The fourth conductive layer may have the same structure as the structure of the third conductive layer described above.

[0102] Next, a cross-sectional structure of a third-layer/fourth-layer contact section 86 is described. The third-layer/fourth-layer contact section 86 is, as shown in FIGS. 13 and 14, formed in a manner to fill a through hole 94 a that is formed in the third interlayer dielectric layer 94. The third-layer/fourth-layer contact section 86 may have the same structure as the structure of the field/second-layer contact section 80 described above. Although not shown in FIG. 13 or FIG. 14, a passivation layer may be formed on the fourth conductive layer.

[0103] [3] Effects

[0104] Effects of the semiconductor device in accordance with the present embodiment will be described below.

[0105] (1) The first drain-gate wiring layer and the second drain-gate wiring layer may be formed in the same conductive layer. However, this structure may make it difficult to reduce the cell area due to the high pattern density of the conductive layer where the first and second drain-gate wiring layers are formed.

[0106] In accordance with a preferred embodiment of the present invention, the first drain-gate wiring layer 30 and the second drain-gate wiring layer 32 are formed in different layers. Specifically, the first drain-gate wiring layer 30 is formed in the first conductive layer, and the second drain-gate wiring layer 32 is formed in the third conductive layer. Since the first drain-gate wiring layer and the second drain-gate wiring layer are not formed in the same layer, the pattern density of the wiring layer can be reduced, accordingly. As a result, with the memory cell in accordance with the present embodiment, the cell area can be reduced.

[0107] (2) In accordance with a preferred embodiment of the present invention, the first drain-gate wiring layer 30 and the second drain-gate wiring layer 32 are formed with linear patterns. As a result, patterning to form the first drain-gate wiring layer 30 and the second drain-gate wiring layer 32 can be readily conducted.

[0108] (3) In accordance with a preferred embodiment of the present invention, the p⁺-type impurity layers 16 d and 17 d that function as well-contact regions are electrically connected to the Vss wiring 64. By this structure, the well potential of the p-well region W20 can be fixed at Vss. As a result, generation of latch up can be suppressed.

[0109] [4] Example of Application of SRAM to Electronic Apparatus

[0110] The SRAM in accordance with the present embodiment may be applied to electronic apparatus, such as mobile equipment. FIG. 15 shows a block diagram of a part of a mobile telephone system. A CPU 540, an SRAM 550 and a DRAM 560 are mutually connected via a bus line. Further, the CPU 540 is connected to a keyboard 510 and an LCD driver 520 via the bus line. The LCD driver 520 is connected to a liquid crystal display section 530 via the bus line. The CPU 540, the SRAM 550 and the DRAM 560 constitute a memory system.

[0111]FIG. 16 shows a perspective view of a mobile telephone 600 that is equipped with the mobile telephone system shown in FIG. 15. The mobile telephone 600 is equipped with a main body section 610 including a keyboard 612, a liquid crystal display section 614, a receiver section 616 and an antenna section 618, and a lid section 620 including a transmitter section 622.

[0112] Although embodiments are described in detail above, the present invention is not limited to the embodiments, and a variety of modifications can be made within the scope of the subject matter of the present invention. Thus, it is intended that the present invention cover modifications and variations that come within the scope of the appended claims and their equivalents. 

What is claimed is:
 1. A semiconductor device having a memory cell including a first load transistor, a second load transistor, a first driver transistor, a second driver transistor, a first transfer transistor, and a second transfer transistor, the semiconductor device comprising: a first gate-gate electrode layer including a gate electrode of the first load transistor and a gate electrode of the first driver transistor; a second gate-gate electrode layer including a gate electrode of the second load transistor and a gate electrode of the second driver transistor; a first drain-drain wiring layer that forms a part of a connection structure that electrically connects a drain of the first load transistor and a drain of the first driver transistor; a second drain-drain wiring layer that forms a part of a connection structure that electrically connects a drain of the second load transistor and a drain of the second driver transistor; a first drain-gate wiring layer that forms a part of a connection structure that electrically connects the first gate-gate electrode layer and the second drain-drain wiring layer; and a second drain-gate wiring layer that forms a part of a connection structure that electrically connects the second gate-gate electrode layer and the first drain-drain wiring layer, wherein the first drain-gate wiring layer and the second drain-gate wiring layer are located in different layers, the first drain-gate wiring layer is located in a layer below the first drain-drain wiring layer, and the second drain-gate wiring layer is located in a layer above the first drain-drain wiring layer.
 2. A semiconductor device according to claim 1, wherein the first gate-gate electrode layer, the second gate-gate electrode layer and the first drain-gate wiring layer are located in a first conductive layer, the first drain-drain wiring layer and the second drain-drain wiring layer are located in a second conductive layer, and the second drain-gate wiring layer is located in a third conductive layer.
 3. A semiconductor device according to claim 2, wherein the second drain-gate wiring layer and the second gate-gate electrode layer are electrically connected to each other through a contact section.
 4. A semiconductor device according to claim 3, further comprising: a first interlayer dielectric layer provided between the first conductive layer and the second conductive layer; a second interlayer dielectric layer provided between the second conductive layer and the third conductive layer, wherein the contact section is provided within a through hole that penetrates the first interlayer dielectric layer and the second interlayer dielectric layer.
 5. A semiconductor device according to claim 2, wherein the second conductive layer is formed from titanium nitride.
 6. A semiconductor device according to claim 1, wherein a plan configuration of the first drain-gate wiring layer and the second drain-gate wiring layer has a linear shape.
 7. A memory system comprising the semiconductor device according to claim
 1. 8. An electronic apparatus comprising the semiconductor device according to claim
 1. 